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  • Author Author: amgalbu
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MagicHat - 14 - Blood pressure measurement

amgalbu

I finally succeeded in measuring blood pressure in an almost reliable way. In this post I will show the results of my experiments

 

In order to determine the law between the value returned by the Force Sensor Resistor and the blood pressure, I run some comparison tests. Basically, I measured my real blood pressure with a normal home medical device (see picture) and associate the readings of the medical devices with the corresponding ADC reading of the FSR.

I made a test on myself (I have a blood pressure of about 130) and my wife (which has a blood pressure of 110)

Here are the data I collected

 

image

 

In terms of accuracy, it seems quite good. I applied the following formula to determine the accuracy of the instrument:

 

   image

 

I got that the blood pressure reading can vary of less the 10, which is not enough for the MagicHat to be approved for use as a diagnostic device, but it's good enough for a gadget. Also, the design can definitely be improved in terms of sensor selection and mechanical building.

Regarding the selection of the sensor, I probably select the wrong one because its range will allow me to measure blood pressure only up to about 150. IN a next design revision a FSR with a maximum load of up to  3 kilos will be selected

 

Given the above data, I have been able to detemine the following linear correlation between ADC reading and blood pressure

 

  image

 

Since I will use Arduino's map function, I need the values corresponding to ADC readings 0 and 1023. So

 

ADC reading = 0 -> Blood pressure = -253,24

ADC reading = 1023 -> Blood pressure = 196,88

 

The above-mentioned results have been obtained in parallel with the choice of the most suitable speed of the elastic rubber string that forces the patient's finger. I need to determine whether the blood is still flowing so I have to wait at least 1.5 seconds to determine whether heart beat is still detectable or not. This requires the movement to be slow. However, it's not very comfortable for the patient if it takes a long time just to measure blood pressure...

The best compromise I found was to move the servo for 20 ms and stop it for 500 ms.

 

Here is a video showing the blood pressure measurement device at work

 

  • in reply to amgalbu

    Hi Ambrogio,

     

    thank you for the replay. I was just curious about this question of the green LED. I will document in-depth later.

     

    Instead, about the USB power from the battery pack, I think it is normal. Using these packs to boost a smartphone, they connects to the normal USB charger plug of the device that I am almost sure it is further filtered by the device with a control power. Did you used a power regulator also with the battery pack ?

     

    Enrico.

  • in reply to balearicdynamics

    Hi enrico

    I designed the biosignal conditioning circuit on a bread board with a stabilized power supply and that worked perfectly (you can see a screenshot of the amplified biosignal in one of my posts). But then I connected one of those battery pack that you typically use to boost the duration of cellular phones. I guess these devices has a low-quality step-up converters inside. With the battery pack plugged in, I saw on the scope a very notable ripple and periodic bursts on the 5V (I can't remember the period of the bursts). The ripple and the bursts affected the working of the opamp and the amplified signal was basically white noise

     

    There are many discussions on the network regarding the best LED color to use. Someone argues that green light propagates better through flesh... anyway I can't provide you any data that confirms of negates this statement

     

    regards

    Ambrogio

  • in reply to amgalbu

    Hi Andy,

    I am documenting in this period due the creation of the prototype of Meditech. I mean that usually it is considered by a medical point of view a more reliable diagnostic the integration of two kinds of readings: one coming from the traditional sphygmomanometer and one from this kind of devices.

     

    Instead, about the kind of senso, I am also oriented to the IR solution instead of the led (sounds strange the adoption of green instead of traditional red). What kind of noise do you have experienced ?

     

    Thanks

  • in reply to balearicdynamics

    Hi Enrico

    I initially built a circuit based on the TTCRT1000, which integrated an IR tx+rx in a very compact plastic package, but I had noise problems I had not been able to work out

    So I moved to an existing solution available at pulsesensor.com that uses  green LED

     

    By a bare diagnostic point of view, a more reliable value acquisition integrates two type of measure in the same moment: sphygmomanometer and finger sensor.

     

    What do you mean here?

     

    cheers

    Ambrogio

  • Be aware that the map function is very useful in many cases, but it is not a math that you control too much in precision and rounding.

     

    Anyway your approach is interesting as it is optimised in the almost limited (for math) Arduino environment. By a bare diagnostic point of view, a more reliable value acquisition integrates two type of measure in the same moment: sphygmomanometer and finger sensor. This last anyway is the most efficient as it can be done very fast and almost in every condition.

     

    What approach did you used to produce the analog signals? Using red light on the finger ?